Curious about this too. At home, I just spray anything that pops up in a mulched area, but I have a client that I did a huge mulch job for last year who is concerned about stopping the weeds that may come through this year.

I rolled out weedblock before applying about 3" of mulch. I would think that should do pretty well, but was wondering if applying "Preen" of something on the mulch now would be a good idea for preventing any seeds that may have settled in from germinating.
Also, I don't know much about poison ivy, but there was a lot of it in the area I mulched. Don't know if it will work its way up through the barrier or not.